Additive manufacturing is increasingly becoming a significant fabrication technique, both in research and industrial settings, applicable to a broad range of applications. Some commercially important additive manufacturing examples include, for example, low-cost rapid tooling manufacturing, low-volume prototype and production runs, medium-volume automotive and aerospace applications, dental restoration, orthopedic implants, custom orthotics, and user-specific artificial limbs. Furthermore, additive manufacturing methods have been used in biomedical research settings to create heterogeneous tissues from individual precursor cell types and to create functional replacements for missing or damaged body parts.
Despite a great deal of effort and the diversity of additive manufacturing techniques, no additive manufacturing process can fabricate high quality electrical interconnections, computational circuits, sensors or actuators in combination with mechanical elements in an integrated component, i.e., electromechanical component.
With Direct-Write electronics (“DW”), or the similar Direct Print (“DP”) technique, it has been demonstrated that inkjet printers are capable of fabricating transistors, and have used a combination of inkjet or digital printing and Fused Deposition Modeling (“FDM”) or Stereolithography (“SLA”) to create electrical circuits within a 3D printed part. One interesting alternative approach uses conventional semiconductor fabrication to create very small semiconductor devices that are subsequently blended with an ink binder. Although electromagnetic actuators fabricated in one process via FDM and DP has been recently demonstrated, it is rudimentary at best.
Despite this progress, enormous challenges must be overcome. Synthesizing electrically conductive materials with volume resistivity similar to bulk metals that can be extruded or deposited in a low-temperature environment (so that is it process-compatible with other materials in the assembly) remains an elusive challenge. The current state of the art, available from various vendors, employs powdered metal inks that are solvent-borne and achieve volume resistivity that is four times (4×) to ten times (10×) larger than bulk metal in the case of silver, and 10× to 50× for copper. These materials require a post-process sintering step, typically by heating to between 80 and 150 degrees Celsius (° C.) in order to achieve the stated resistivity, which can be difficult to integrate with other heat-sensitive components within the assembly. The active devices such as transistors that have been fabricated thus far have lower carrier mobility and lower on-off ratios than similar devices fabricated in silicon.
Printed transistors using complementary logic (p-channel and n-channel devices) have been combined to produce ring-oscillators, inverters and NAND (“Not AND”) gates; however low fabrication-temperature organic transistors rely on semiconductor materials that offer dramatically lower charge carrier mobilities than are available via conventional semiconductor fabrication techniques. The large discrepancy between the highest reported electron and hole mobilities for printed polymers for organic transistors imposes additional design trade-offs when fabricating complementary transistor circuits, a key component of logic circuits.
In certain situations carrier mobility impacts the drain current and the transconductance in a field effect transistor. Larger drain currents are desirable for some applications; however, as a consequence of lower mobility and larger oxide thickness, printed organic transistors typically offer drain currents that are several orders of magnitude smaller than conventional devices. It has been found that wider channels can be used to increase drain current, though this is usually accompanied by increased leakage. Resolution limitations of current printable electronics techniques impose a feature-size penalty of nearly three orders of magnitude, relative to conventional semiconductor fabrication techniques, which limits the amount by which the channel length can be reduced.
Larger transistor feature sizes lead to increased parasitic capacitances at each transistor, reducing their switching speed. Lower transconductance also limits switching speed; the propagation delay of recent fast organic transistors is at least three orders of magnitude slower than conventional transistors, limiting their use to relatively simple logic circuits since this delay accumulates with each cascaded logic cell. Printed organic semiconductors sacrifice endurance relative to conventionally fabricated circuits, with published shelf- and operating lifetimes ranging from several weeks to two years.
Conductor quality in printed electronics is impaired by incompatible material processing requirements. Low-resistivity base materials and narrow traces with high current-carrying capacity are desired In order to achieve favorable conductivity, electrically conductive materials are used. Electrically conductive materials including conductive materials that can be inkjet-printed or extruded are referred to as “inks”. These inks typically require a post-print curing or sintering step that entails heat-treating at temperatures ranging from 125 to 500° C. for an extended period of time. Since this range exceeds the glass-transition temperature of most common polymers used in additive manufacturing, the sintering step can cause other materials in the part to melt or degrade.
To circumvent this problem, alternative sintering techniques have been developed based on chemical reactions, resistive heating, plasma, photonic energy, and radio-frequency heating. Recent results compatible with low-temperature polymer substrates demonstrate conductor resistivity of 2-10× bulk via pulsed Xenon lamps, and pulsed-laser, though integration of these methods with structural additive manufacturing materials has not been demonstrated. Reactive silver inks have been shown to yield traces with conductivity nearly equal to bulk silver after 15 minutes of sintering at 90° C., though material costs may limit this approach.
A commercially available method for creating electrical conductors on the surface of plastic parts, known as Laser Direct Structuring (“LDS”), uses a laser to ablate the thermoplastic substrate where conductive traces are desired; organic-metallic additives in the plastic are activated during this process, leaving behind a surface that can be plated during successive wet metallization steps. However, like the other methods mentioned above, LDS creates electrical traces only on the surface of a part and limitations in achievable trace thickness impose constraints on current-carrying capacity despite continuing improvements in material resistivity.
An alternative fabrication approach, Shape Deposition Manufacturing (“SDM”), circumvents material and process compatibility issues by embedding prefabricated components into an assembly as it is being fabricated. This concept has been demonstrated by embedding complete assembled circuit boards as well as discrete components; these components are interconnected with embedded wires or printed conductors subject to the limitations discussed above. At a smaller scale, individual pieces of prefabricated semiconducting material referred to as “chiplets”, have been self-assembled to form functional arrays of devices over large scales, including roll-roll manufactured LED sheets, and flexible arrays of chip-scale solar cells. When fabricated with high-speed electrical interconnects on their edges, individual chiplets can be interconnected to form larger composite circuit “Quilts”. A related approach also decomposes the problem into separate high-temperature fabrication steps using conventional micro-fabrication tools, followed by a low-temperature assembly process based on transfer printing.
Existing manufacturing methods exist that embed components; however these methods rely on special-purpose embedding of particular components for specific designs. For example, U.S. Pat. No. 5,278,442 to Prinz et al. discloses electronic components formed in place by incremental material build-up of thin layers. At least one mask is used per layer to form electronic components made of conductors such as gold and copper, insulators such as ceramic materials and possibly semiconductors, all of which are applied by thermal deposition spray using a thermal deposition spray.
Another example of an existing manufacturing method that embeds components is described in U.S. Pat. No. 5,301,415 to Prinz et al., which forms three-dimensional objects by applying segments of complementary material and deposition material so as to form layers of material. Selected segments of material are then shaped after one or more segment is formed. In this manner, layers of material form a block containing the object made of deposition material and surrounded by complementary material, which may subsequently be removed.
As further described in U.S. Pat. No. 5,286,573 to Prinz et al., the support structure has a melting point lower than the melting point of the deposition material so that the support structure can be removed by a melting process.
With all of the above described methods, embedded components are printed or shaped within a complementary material, which may ultimately be removed. These components are specific to the desired application of the assembly. Though multiple additive material deposition techniques have been developed to address diverse users, existing techniques fail to address three critical requirements that electromechanical printers must satisfy. First, existing methods produce components with electrical performance that is inferior to conventionally produced electrical components by several orders of magnitude. Second, existing methods are incapable of combining the diverse materials required for complex, integrated electromechanical systems. Thus, there is a need for general-purpose, scalable manufacturing methods that employ a library of pre-fabricated modular components that are universal in manufacturing a variety of assemblies. Third, existing techniques that embed components do not contemplate or demonstrate a modular, general-purpose system. Instead they embed specific pre-fabricated components that are unique to each intended end-use or printed/assembled design. This limitation makes existing methods incapable of addressing the need for a general-purpose electromechanical 3D printer. The invention satisfies these needs.